So what: While the offer represents just an 18% premium over Illumina's closing price on Tuesday, it's about 65% higher than it was in December when rumors first started swirling about Roche's interest. Roche is making the move to boost its presence in the growing diagnostics business, but given that Illumina shares are soaring about 20% past the bid, it will likely need to make a higher offer to make it happen.

Now what: When you make 40% in one morning, taking at least some dough off the table seems like the prudent thing to do. While a higher bid from Roche or another large player is likely, holding out for a dramatically better offer seems a bit risky. Given Illumina's heavy reliance on dwindling government grants, this seems like an opportune time to lighten up the position.